COMMITMENTS AND CONTINGENCIES, Environmental (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012
NPC [Member]
Environmental [Abstract]
Reclamation estimate of coal washing and load-out facility site	$ 5
SPPC [Member]
Environmental [Abstract]
Third party ownership percentage in Valmy Generating Station (in hundredths)	50.00%

Estimated amount, supported by a bond, of a reclamation liability that is associated with a coal washing and load-out facility site in Wellington, Utah.

No definition available.

Name:	nve_ReclamationEstimateOfCoalWashingAndLoadOutFacilitySite
Namespace Prefix:	nve_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant